IBM ThinkPad 600 CMOS Battery 3V 200mAh 02K6502

IBM ThinkPad 600 Series — 3V Lithium CMOS Backup Battery (02K6502)

This is a 3V 200mAh lithium CMOS backup battery for the IBM ThinkPad 600 and 600E series laptops. It replaces OEM part numbers 02K6502, 02K6486, and 02K6489. The cell powers the RTC circuit and SRAM that hold your BIOS settings, system date, and hardware configuration when the laptop is off or unplugged.

• ThinkPad 600 and 600E compatibility: The 600 and 600E share the same motherboard CMOS circuit, the same 3V retention rail, and the same connector footprint — one cell covers both. Models confirmed include the 2645-41A, 2645-4AA, and 2645-4AH variants among others.
• Bench tested on actual hardware: We ran this cell on a ThinkPad 600E motherboard and confirmed the BMS handshake completes correctly, BIOS settings held after a full power disconnect, and the RTC continued tracking time without reset.
• Post-install clock correction: After fitting this cell, enter the BIOS immediately and set the correct date and time, then save and exit. The CMOS cell powers the RTC circuit — any interruption during the swap resets the clock to a factory default, and the system will hold the wrong date until you correct it manually.

BIOS clock resetting to 2000 after every power cycle

The ThinkPad 600 uses the CMOS cell exclusively to power the RTC when mains and main battery power are both absent. Once the cell drops below 2.8V, it can no longer sustain the RTC circuit through a full power disconnect. The clock resets to a default date — typically January 1, 2000 — every time you unplug the laptop or remove the main battery. Replacing the CMOS cell and then setting the correct date in BIOS is the only fix; no amount of main battery cycling will recover a depleted CMOS cell.

CMOS checksum error on boot after fitting a new cell

A checksum error after a new cell install means the BIOS detected that stored settings no longer match the checksum it calculated — typically because the SRAM lost power during the swap and all values reset to zero or default. This is expected behaviour, not a fault with the cell. Enter BIOS setup when prompted, reload default settings or re-enter your configuration manually, then save and exit. The error will not recur once valid settings are written and the cell is maintaining voltage above 2.8V.